1. Field of the Invention
This invention relates generally to vision correction by spectacles and eyeglasses and more specifically to eye contact type ophthalmic lenses.
2. Description of the Prior Art
Scleral and semi-scleral contact lenses have long been known in the art. Scleral lenses which are supported by the sclera and which arch over the cornea are described, for example, in U.S. Pat. Nos. 2,129,305; 2,129,304; 2,178,873; 2,196,066; and 2,438,743 to Feinbloom; U.S. Pat. No. 2,330,837 to Mullen; and U.S. Pat. Nos. 3,781,096 and 3,937,566 to Townsley.
Scleral lenses have not been in popular use recently. Their design does not normally allow for movement of the lens on the eye, and it is now known that some such movement is desirable. Scleral lenses are usually made of an oxygen impermeable material and thus block the supply of atmospheric oxygen to a large area of the eye.
Fertsch et al disclose in U.S. Pat. No. 1,921,972 a scleral lens having its inner surface formed from two spherical sections, each section having a different radius of curvature and spherical center. The sharp edge where the two sections meet is smoothed by forming a third spherical cut which can have an infinite radius of curvature, i.e., a conical section. The use of a conical section between the two spherical sections of a scleral lens, as taught by Fertsch, does not take into recognition the topography of the individual eyeball and does not result in central and edge contact with tearfilm clearance therebetween or in ready tearfilm exchange with blinking.
Flush-fitting semi-scleral lenses which are in total contact with the cornea are described in U.S. Pat. No. Re. 29,229 to Girard et al.
Girard et al also discloses a semi-scleral lens which arches over the cornea. Girard et al does not disclose a semi-scleral lens which is shaped relative to the topography of an individual eyeball. Such semi-scleral lenses, like corneal lenses, often are given to excessive movement on the eye due to gravity and eyelid drag. Such movement is commonly referred to as lag and causes variable vision upon blinking. Such variable vision can be distracting when it is excessive.
Tuohy indicates in U.S. Pat. No. 2,510,438 that contact lenses have been produced which have portions resting directly on the cornea and a supporting flange or border resting on the scleral portion of the eye. Tuohy points out that such lenses are difficult to fit accurately and that the flange may seal off the corneal area from the flow of eye fluids. Touhy's solution to the problem is a corneal lens which contacts the central cornea and which has a slightly raised marginal portion. A semi-scleral contact lens which is easy to fit and which does not seal off the corneal area from the flow of eye fluids is desirable.
Semi-scleral contact lens have been made from relatively rigid materials such as polymethylmethacrylate (PMMA) in shapes which do not take into account the topography of the individual eye. Such lenses have exhibited some usefulness although they are subject to the disadvantages pointed out by Touhy.
The semi-scleral lens design used with relatively rigid materials cannot normally be used with certain resilient materials such as silicone rubbers, hydratable gels and hydroxyethylmethacrylate (HEMA). Silicone rubbers are especially desirable lens materials because of their oxygen permeability and their bio-compatibility. When the semi-scleral hard lens design is used with such resilient materials, the blinking action of the eyelid sometimes causes the lens to suction to the eyeball. A design which will enable resilient materials such as silicone rubbers to be used as semi-scleral lenses is desirable.